The present invention relates to a process for the treatment of cellulose substrates which makes it possible to reduce the penetration of adhesive-resistant agents into the substrates and/or to decrease their sensitivity to water, by bringing these cellulose substrates into contact with aqueous emulsions based on grafted functionalized polyorganosiloxane, and to the use of said aqueous emulsions as adhesive-resistant and/or water-repellent coating on cellulose substrates in the manufacture of pressure-sensitive adhesives.
Self-adhering products or pressure-sensitive adhesives generally comprise a frontal material composed of a paper or of a polymer film, such as of PVC, PBE, PP or PET, this part of the product generally being printed and acting as label, which will subsequently be adhesively bonded to the article to be identified. The adhesive bonding of the label is carried out on the article by simply pressing a layer of pressure-sensitive adhesive applied to the back of the label. Protection of the adhesive or sticky layer of the self-adhesive label, at the time when the latter is applied to the product concerned, is provided by a protective paper or polymer film to which has been applied a fine silicone layer, usually in the form of a deposit of 1 to 2 g/m2. The silicone layer which is brought into contact with the adhesive layer makes it possible to easily remove the protective paper or film at the time of use of the self-adhering label. The force which will have to be exerted to separate the two constituents is known as the release force. This force must be controlled for the majority of label applications, in which the labels are applied automatically, and must remain stable over time.
A large proportion of self-adhesive complexes are available in the form of rolls and use, as protective film or paper, a paper of the glassine type with a density of 60-65 g/m2. It is a paper manufactured from a highly beaten bisulfite pulp. The exhaustive beating makes possible significant entangling of the fibers and contributes to xe2x80x9cclosing upxe2x80x9d the paper. To further reduce the penetration of the silicone which will subsequently be deposited on the surface of the glassine paper, a solution of carboxymethylcellulose/polyvinyl alcohol (CMC/PVA) is applied to both faces of the paper with the size press of the papermaking machine. The paper is subsequently calendered off the machine. This final operation puts the finishing touches to the xe2x80x9cclosing upxe2x80x9d of the paper and confers a degree of transparency on the latter. This transparency is necessary when photoelectric cells are used to provide for automatic control of the positioning or of the progression of the paper web on various machines, such as printing machines or applicator machines.
Other protective papers are used in the field of self-adhesive products and in particular papers coated with kaolin. The inorganic coating makes possible good surface maintenance of the silicone deposited (which is not the case for a paper of the glassine type) but does not exhibit the advantage of the transparency of glassine. The protective paper of the type coated with kaolin will thus, in the majority of cases, be used in the form of sheets in applications of a more manual nature.
However, standard glassine paper coated with carboxymethylcellulose/polyvinyl alcohol exhibits certain disadvantages, including:
a large amount of silicone has to be deposited on its surface to provide the desired release force with respect to a given adhesive layer. This is because a preliminary study of the profile of penetration of a silicone through a paper carried out by analyzing the section of the paper using a scanning electron microscope coupled to an X-ray dispersive energy analyzer made it possible to establish that approximately 60% of the silicone applied to a glassine paper is found in the body of the paper (depth of penetration of up to 11 microns) and thus only the remaining 40% are of use in providing the desired release value.
the standard CMC/PVA coating leads, under certain conditions (hydrometric or abrasive conditions), to fluffing on the machine.
very poor dimensional stability. In fact, because of its highly beaten and strongly calendered nature, glassine paper varies dimensionally in a very significant fashion with hygrometric variations in the surroundings in which it is moved and thus develops very significant curl when it is complexed in the form of a self-adhesive product with a frontal product generally exhibiting better dimensional stability toward moisture. The protective frontal components in fact form a kind of xe2x80x9cdouble stripxe2x80x9d which reacts differentially toward variations in humidity. In fact, the curl often prevents the adhesive product from being satisfactorily used, for example when it is passed through photocopiers, laser printers or ink jet printers. Furthermore, if a standard glassine directly receives water on its surface, it instantaneously develops a curl such that the sheet of paper will roll up on itself.
In view of the abovementioned disadvantages, it is consequently difficult to conceive of using this type of cellulose substrate for silicone treatment in an aqueous base if the silicone coating machines are not entirely suitable for this type of silicone treatment, such as insufficient web tension, thermal ovens with a flat outline and with hot air being blown into the ovens, which are not of the flotation type.
It should also be noted that Abstract XP-002136528 (corresponding to Patent Application JP 06 248244A) is completely different from the subject matter of the present invention in that it relates to the preparation of a release paper of the Kraft type pretreated with silicone which does not exhibit specific optical properties and thus does not relate to the treatment of a cellulose substrate of the glassine type which has not been pretreated with silicone. Furthermore, the viscosity modifier added to the emulsion, which emulsion is used for this purpose, acts solely as thickener.
One of the aims of the present invention consequently consists in overcoming the abovementioned disadvantages and in providing a process for the treatment of a cellulose substrate and in particular of a paper of the glassine type which makes it possible to reduce the penetration of adhesive-resistant agent or agents into the substrate and/or to decrease its sensitivity to water by bringing the cellulose substrate into contact with an aqueous emulsion based on a grafted functionalized polyorganosiloxane.
To this end, according to the present invention, the aqueous emulsion based on a grafted functionalized polyorganosiloxane furthermore comprises a polymeric material of the acrylic type.
The polymeric material of the acrylic type advantageously originates from an acrylic monomer chosen from the group consisting of acrylic acid, methacrylic acid, acrylamide, methacrylamide, methyl acrylate and methacrylate, ethyl acrylate and methacrylate, propyl acrylate and methacrylate, butyl acrylate and methacrylate, hexyl acrylate and methacrylate, cyclohexyl acrylate and methacrylate, benzyl acrylate and methacrylate, isopropyl acrylate and methacrylate, octyl acrylate and methacrylate, methyl xcex1-chloroacrylate, ethyl xcex1-ethoxyacrylate, 2-ethylhexyl acrylate and methacrylate, phenyl acrylate and methacrylate, xcex1-chloroacrylonitrile, N,N-dimethylacrylamide, N,N-dibenzylacrylamide, hydroxyethyl acrylates and methacrylates, and mixtures of at least two of these acrylic monomers.
According to an advantageous embodiment of the invention, the polymeric material of the acrylic type comprises a styrene fraction and is an acrylic/styrene copolymer.
According to an advantageous embodiment of the invention, the acrylic/styrene copolymer is chosen from the group consisting of copolymers of acrylic or methacrylic acid and of styrene, copolymers of butyl acrylate or methacrylate and of styrene and copolymers of methyl acrylate or methacrylate, of acrylonitrile or methacrylonitrile, of styrene and of 2-ethylhexyl acrylate or methacrylate.
The invention also relates to the aqueous emulsion of polyorganosiloxane and of acrylic polymeric material mentioned above as adhesive-resistant and/or water-repellent coating on a cellulose substrate, in particular glassine paper, in the manufacture of pressure-sensitive adhesives.
Other details and distinctive features of the invention will emerge from the description given below as nonlimiting example of several specific embodiments of the invention.
As has already been indicated above, the cellulose substrate is treated by applying to it, on one face or on both faces, a layer of aqueous emulsion based on a grafted functionalized polyorganosiloxane and on a polymeric material of the acrylic type. In fact, the expression xe2x80x9cpolymeric material of the acrylic typexe2x80x9d is understood to mean any polymer or copolymer material comprising acrylic. For this, use will be made of the polymeric materials obtained by polymerizing an acrylic monomer and more particularly acrylic acid, methacrylic acid, acrylamide, methacrylamide, methyl acrylate or methacrylate, ethyl acrylate or methacrylate, propyl acrylate or methacrylate, butyl acrylate or methacrylate, hexyl acrylate or methacrylate, cyclohexyl acrylate or methacrylate, benzyl acrylate or methacrylate, isopropyl acrylate or methacrylate, octyl acrylate or methacrylate, methyl xcex1-chloroacrylate, ethyl xcex1-ethoxyacrylate, 2-ethylhexyl acrylate or methacrylate, phenyl acrylate or methacrylate, xcex1-chloroacrylonitrile, N,N-dimethylacrylamide, N,N-dibenzylacrylamide, hydroxyethyl and butyl acrylates and methacrylates, and mixtures of at least two of these acrylic monomers. The polymeric material of the acrylic type will advantageously also comprise a styrene fraction. The expression xe2x80x9cstyrene fractionxe2x80x9d is understood to mean any monomer comprising styrene which can copolymerize with the acrylic fraction to produce an acrylic/styrene copolymer. In this respect, styrene, xcex1-methylstyrene, methylstyrene, 2,4-dimethylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, phenylstyrene, cyclohexylstyrene, benzylstyrene, chlorostyrene, 2,5-dichlorostyrene, bromostyrene, cyanostyrene, nitrostyrene, N,N-dimethylaminostyrene, acetoxystyrene, phenoxystyrene and mixtures of at least two of these styrene monomers are particularly well suited as styrene monomer. Acrylic/styrene copolymers which are particularly well suited are copolymers of (meth)acrylic acid and of styrene, copolymers of butyl meth(acrylate) and of styrene and copolymers of methyl meth(acrylate), of meth(acrylonitrile), of styrene and of 2-ethylhexyl meth(acrylate), the expressions xe2x80x9c(meth)acrylicxe2x80x9d, xe2x80x9cmeth(acrylate)xe2x80x9d and xe2x80x9cmeth(acrylonitrile)xe2x80x9d covering both acrylic and methacrylic materials.
As regards the grafted functionalized polyorganosiloxane component of the aqueous emulsion, reference will be made more particularly to Patent Application EP-0 731 137, which discloses in detail the formulation and preparation of this family of products. As will be noted, these grafted functionalized polyorganosiloxanes (POSs) are in fact composed of grafted copolymer units originating from at least one ethylenically unsaturated monomer (a) which can polymerize by the radical route and of a functionalized polyorganosiloxane comprising alike or different units of the following formula:
RaYbXcSiO(4xe2x88x92axe2x88x92bxe2x88x92c)/2
in which the various symbols R, X, Y and a, b and c are defined as follows:
R is C1xe2x95x90C18 alkyl, C2-C20 alkenyl or C6-C12 aryl, optionally substituted with halogen;
X is a reactive function linked to Si by Sixe2x80x94C or Sixe2x80x94Cxe2x80x94O bond;
Y is ethylenically unsaturated hydrocarbon group, optionally containing O or N heteroatom, linked to Si by Sixe2x80x94C bond and capable to react radically with the unsaturated monomer (a); a, b and c are 0 to 3 and a+b+c=3; the content of SiO4/2 is less than 30%. Preferably, the units (I) containing X and/or Y are present in the amount such that the functionalized polyorganosiloxane comprises at least 1 milliequivalent, preferably 5 to 500 milliequivalents, most preferably 5 to 100 milliequivalents of X, and at least 1 milliequivalent, preferably 5 to 500 milliequivalents, most preferably 5 to 100 milliequivalents of Y per 100 grams of the functionalized polyorganosiloxane. Preferred reactive function X is a C1-C20 hydrocarbon epoxy-functional group.
According to the present invention, the amounts by weight of polymeric material of the acrylic type and of grafted functionalized polyorganosiloxane in the aqueous emulsion vary respectively between 5 and 20%, preferably from to 20%, and between 10 and 20%, preferably of the order of 15%. As regards the respective amounts of ethylenically unsaturated monomer and of grafted functionalized polyorganosiloxane, these amounts correspond to the ratios by weight specified in Patent Application EP-0 731 737 and are 98-50/2-50 and preferably 95-75/5-25. The aqueous emulsion is in fact obtained by simple mixing of the two polymers in the form of an aqueous emulsion. In this respect, the sizes of the particles of the emulsions used in the mixture are extremely important. They range from a mean particle size of 0.3 to 1.0 xcexcm and advantageously of 0.6 xcexcm for the functionalized polyorganosiloxane and of 0.05 to 0.3 xcexcm, advantageously of 0.15 xcexcm, for the acrylic polymeric material. In fact, this very marked difference in size makes it possible to achieve better filling of the free spaces during spreading of the aqueous emulsion over the cellulose substrate, the small particles occupying the spaces created between the large particles. This configuration promotes the formation of a homogeneous and closed film at the surface of the cellulose substrate during the drying of the emulsion after coating and consequently makes it possible to decrease the penetration into the paper of the silicone which will subsequently be deposited and the absorption of water by the cellulose on the side of the coating thus applied. This advantage is demonstrated in particular by the appended FIGS. 2 and 3, which give the water absorption profile and the silicone absorption profile at 25xc2x0 C. of two coated papers and of a noncoated reference paper, which show that the barrier to water and to a lesser extent to silicone is conferred in particular by the copolymer of butyl acrylate and of styrene which has been added to the polyorganosiloxane. It should also be noted in this respect that the acrylic copolymer emulsion is selected for its pH, compatible with that of the grafted functionalized POS. FIG. 4 is a graph showing the determination by thermogravimetry of the permeability to water vapor of various papers, namely of a conventional calendered paper with a CMC/PVA coating (A), of a base paper before the barrier-forming layer has been applied (B), of a base paper with a barrier layer based on a solution of grafted functionalized POS and of butyl acrylate/styrene copolymer of the invention (C) and of a base paper with a barrier layer based on a solution of the invention and a silicone layer (D).
The thermogravimetric method used consists in depositing, in a glass crucible, a predetermined amount of water and in covering this crucible using the paper, the permeability of which it is desired to evaluate. As the contact surface between the water vapor and the paper is always identical (equal area) for all the samples studied, the loss in weight as a function of time is plotted at a constant preset temperature (60xc2x0 C.). It is observed, from the four curves on the graph, that the paper with the barrier layer of butyl acrylate/styrene copolymer of the invention is more effective in slowing down the transmission of the water vapor in comparison with the other papers and in particular with the paper coated with a conventional CMC/PVA barrier layer. The permeabilities of the various papers were calculated by using the following formula:   P  =            Δ      ⁢              xe2x80x83            ⁢      W      xc3x97      e              A      xc3x97              P        VAP            xc3x97      t      xc3x97      M      ⁢              xe2x80x83            ⁢      M      
where
P=permeability (mol.mxe2x88x921.sxe2x88x921.Paxe2x88x921)
xcex94W=loss in weight
e=thickness
A=area of the contact surface
PVAP=vapor pressure of water at 60xc2x0 C.
t=time for bringing into contact
MM=molar mass of water.
xe2x80x831/P=xcfx861/P1+xcfx862/P2
where
xcfx86 is the fraction by volume of the barrier layer n
1/P3=(58/60)/P2+(2/60)/Pinvention 
⇄Pinvention=(30(1/P3xe2x88x920.97/P2))xe2x88x921 
⇄Pinvention=8.3989Exe2x88x9213 mol.mxe2x88x921.sxe2x88x921.Paxe2x88x921 
1/P4=(58/61)/P2+(2/61)/Pinvention+(1/61)/Pinvention+silicone 
⇄Pinvention+silicone=(61 (1/P4xe2x88x92(58/61)/P2xe2x88x92(2/61)/Pinvention))xe2x88x921 
⇄Pinvention+silicone=5.2358Exe2x88x9213 mol.mxe2x88x921.sxe2x88x921.Pa31 1 
It should also be noted that the glass transition temperature of the copolymer of butyl acrylate and of styrene used in combination with the POS is 20% higher than that of the grafted functionalized polyorganosiloxane polymer used alone.
Tg: butyl acrylate/styrene+POS: 38xc2x0 C.
Tg: grafted functionalized polyorganosiloxane: 18.5xc2x0 C.
This difference in glass transition temperature is also extremely advantageous in the sense that it contributes, to the mixture of the invention, a markedly less sticky nature of the dry coating deposited at the surface of the paper than during the use of the POS alone. This property is very important with respect to paper rolls as it makes it possible to prevent the turns of paper from sticking under the effect of the reeling tension and in particular with respect to the calendering of freshly coated paper on the papermaking machine. This is because the coated paper is subsequently calendered on a supercalender off the machine under high pressure and at high temperature and it is essential to prevent any sticking of the paper in the calender if breaking of the paper is to be avoided, which breaking will thus result in a loss of time, extensive cleaning and a considerable financial loss. The graph in FIG. 5 shows in this respect the change in the glass transition temperature of a layer of polyorganosiloxane polymer as a function of the percentage of butyl acrylate/styrene copolymer added, the Tg of the mixture being calculated by Fox""s law.
As in the case of conventional coatings on a cellulose substrate, the aqueous emulsion of grafted functionalized polyorganosiloxane and of acrylic polymeric material can comprise one or more additives such as thickeners, antifoaming agents and/or waxes. Thus it is that, in the case of an application with a size press, a thickener will be added with the aim of obtaining a viscosity similar to that of a conventional surface treatment of glassine paper, namely CMC/PVA. The addition of a wax may prove to be necessary if the manufacture of the paper coated with the barrier layer is followed by a calendering operation of the machine. Taking into account the coating conditions, such as the temperature, the shear forces or the turbulence, the use of an antifoaming agent may also prove to be useful. In this respect, mention will more particularly be made, as thickener, of an aqueous dispersion of poly(ammonium acrylate) and, as antifoaming agent, of a modified polyalkoxylated alkyl ether in a paraffinic medium. The total amount by weight of the additive or additives in the emulsion according to the invention is generally of the order of 0.3 to 1.5%.